Pesticides often have short effective lifetimes on target sitesbecause they encounter various problems such as volatilization,leaching, and degradation. Enhancement of the pesticidal efficacyon target sites requires repeated pesticide applications, which isundesirable because of high cost, possible phytotoxicity, and,more importantly, certain pesticides are well-known as environ-mental pollutants (1). Hence, controlled-release formulations(CRFs) of pesticides have gained great interest because theyallow usage ofminimumamounts of pesticide for the sameperiodof activity, thereby decreasing the risk to the environment (2).
CRFs developed so far are based on either physical encapsula-tion of active agentswith polymericmaterials, in which the polymeracts as a rate-controlling device or chemical combinations, in whichthe polymer acts as carrier for the agent. In the case of chemicalcombination, the active agent is chemically attached to a natural orsynthetic polymer by a specific chemical bond, via either an ionic orcovalent linkage. The release of the active agent is then primarilydependent on the rate of cleavage of the polymer-active agentbonds (3).
Recently, CRFs for pesticides based on chemical combinationhave received great interest because they offer safer, more efficient,and more economical crop protection (4). To date, controlledrelease of pesticides based on chemical combination is carried outby either hydrolytic or microbial cleavage of the chemical linkagebetween the pesticide and the polymeric backbone (5). To the best
of our knowledge, there has been no report on cleavage of thepolymer-pesticide bonds by light. This prompted us to develop anew type of formulation based on chemical combination utilizinglight for the controlled release of pesticides. Furthermore, we werealso interested in designing a delivery device that not only acts as acarrier for the pesticides but also possesses pesticidal activity after itreleases the pesticides.
In recent years, use of photoremovable protecting groups(PRPGs) for the release of active molecules has become thesubject of expanding interest because it allows both spatial andtemporal control over the release (6). PRPGs are functional groupswhich are used to cage an active molecule in such a way that theactivity of the molecule is masked. Later, exposure to light releasesthe protecting group, restoring functionality to themolecule.On thebasis of the above strategy, several bioactive molecules includingnucleic acids (7), amino acids, enzyme substrates, and catalysts ofbiochemical reactions (8) were chemically caged using PRPGsand released on irradiation by using a UV-visible light source.Recently, Gudmundsdottir’s group also demonstrated the con-trolled release of fragrances underUV light over an extendedperiodof time by chemically caging volatile alcohols such as geraniol usinga PRPG (9).
Among the PRPGs, some of the groups are fluorescent andhave greater advantage over nonfluorescent protecting groupsbecause they not only release molecules of interest at desiredlocations for a specific period of time but also allow us to visualize,quantify, and follow the spatial distribution, localization, anddepletion of the caged compounds by using techniques far moresensitive thanUV(10-14).Consequently, this encouragedus touse
fluorescent PRPG as a delivery device for controlled release ofpesticides because in addition to its controlled release, it will alsoenable detection of caged pesticide residues at low level.
Herein, we report a new controlled-release formulation inwhich 2,4-dichlorophenoxyacetic acid (2,4-D) is chemically cagedby fluorescent PRPGs of coumarin derivatives and later uncagedusing UV-vis light. For the present study, we selected 2,4-Dbecause it is a widely used herbicide and, more importantly,because it has a high leaching potential, which poses a threat tosurface and groundwater contamination (15, 16). In the case ofPRPGs, coumarin-based protecting groups were preferred forcaging 2,4-D due to their unique properties, such as (i) they allowthe rate of photorelease, absorptionmaxima, and the solubility oftheir caged compounds to be altered by having suitable sub-stituent at 6/7-position of their moiety (12); (ii) most of thecoumarin derivatives exhibit strong fluorescence properties (17);and (iii) more importantly, compounds having a coumarinskeleton are known to possess pesticidal activity (18).
MATERIALS AND METHODS
Chemicals. Ethyl acetoacetate, 2,4-D, resorcinol, molecular bromine,formaldehyde, acetaldehyde, 2-[4-(2-hydroxyethyl)-1-piperazine]ethanesul-fonic acid (HEPES) buffer, acetyl chloride, acetonitrile, and methanol werepurchased from Merck. Dimethyl sulfate and ethyl chloroformate werepurchased from Spectrochem Pvt. Ltd. m-Amino phenols and DMSO-d6were purchased from Sigma-Aldrich. All of the chemicals received were ofanalytical grade and used without further purification. Double-distilledwater was used in this experiment. All of the stock solutions were kept in therefrigerator prior to use.
Instrumentation. 1H NMR (400 MHz) spectra were recorded on aBruker-AC 400 MHz spectrometer. Chemical shifts are reported in partspermillion from tetramethylsilanewith the solvent resonance as the internalstandard (deuterodimethyl sulfoxide, 2.54 ppm). Data are reported asfollows: chemical shifts, multiplicity (s, singlet; d, doublet; t, triplet; m,multiplet), coupling constant (hertz). 13C NMR (100 MHz) spectra wererecorded on a Bruker-AC 400 MHz spectrometer with complete protondecoupling. Chemical shifts are reported in parts per million from tetra-methylsilane with the solvent resonance as the internal standard (deutero-dimethyl sulfoxide, 40.45 ppm). Chromatographic purification was donewith 60-120 mesh silica gel (Merck). For reaction monitoring, precoatedsilica gel 60 F254 TLC sheets (Merck) were used. UV-vis absorptionspectra were recorded on a Shimadzu UV-2450 UV-vis spectropho-tometer, and fluorescence emission spectra were recorded on a HitachiF-7000 fluorescence spectrophotometer. FT-IR spectra were recorded on aPerkin-Elmer RXI spectrometer. High-resolution mass spectra were re-corded using a Qtof Micro YA263 mass spectrometer. RP-HPLC wasperformed using Waters 2489 liquid chromatography on a C18 column(4.6 mm � 250 mm) with a UV-vis detector. Photolysis of all the cagedcompounds was carried out using a 125Wmedium-pressure mercury lampsupplied by SAIC (India).
Methods. General Procedure for the Synthesis of the Caged Com-pounds (2a-g). 4-(Bromomethyl)-7-substituted coumarin 1a-g (1 equiv)was dissolved in dry N,N dimethylformamide (DMF) (2 mL). To the solu-tion were added potassium iodide (2 equiv), potassium carbonate (2 equiv),and 2,4-D (1 equiv). The reaction mixture was stirred at 55 �C for 3 h. Aftercompletionof the reaction, solventwas removedunder vacuum.To the cruderesidue was added ethyl acetate (EtOAc), followed by washing with brinewater. The organic layer was collected, dried over Na2SO4, and evaporatedunder vacuumtoyield a reddishbrown residue,whichwas further purifiedbycolumn chromatography using eluant 20% EtOAc in n-hexane.
Photophysical Properties of Caged Compounds of 2,4-D (2a-g).The UV/vis absorption and emission spectra of degassed 3 � 10-5 Msolution of caged compounds (2a-g) in MeOH:HEPES (80:20) were
11846 J. Agric. Food Chem., Vol. 58, No. 22, 2010 Atta et al.
recorded. The Stokes’ shift has been calculated from the difference in theabsorption and the emissionmaxima of the caged compounds. Fluorescencequantum yield of the caged compounds was calculated using the eq 1.
ðΦf ÞCG ¼ ðΦf ÞSTðGradCGÞðGradSTÞ
ðη2CGÞðη2STÞ
ð1Þ
where the subscripts CG and ST denote caged compound and standard,respectively. Quinine sulfate in 0.1 (N) H2SO4 solutions was taken asstandard (19). Φf is fluorescence quantum yield, Grad is the gradient fromthe plot of integrated fluorescence intensity vs absorbance, and η therefractive index of the solvent.
DeprotectionPhotolysis ofCagedCompounds of 2,4-D (2a-g).Asolution of 10-5 M of the caged compound (2a-g) was prepared inMeOH/HEPES buffer (80:20). Half of the solution was kept in the darkand to the remaining half was passed nitrogen followed by irradiation atdifferentUVwavelengths (310, 350, and 410 nm) individually, using a 125Wmedium-pressureHg lamp filteredby suitable filterswith continuous stirring.At regular intervals of time, 20 μL aliquots were taken and analyzed by RP-HPLC using a mobile phase of acetonitrile/water (8:1), at a flow rate of 1mL/min (detection,UV254 nm). Peak areaswere determined byRP-HPLC,which indicated gradual decrease of the caged compound with time, and theaverageof three runs. The reactionwas followeduntil the consumptionof thecaged compound was <5% of the initial area.
On the basis of HPLC data for each caged compound, the naturallogarithm of the concentration of caged compound (ln C) versus irradia-tion time was plotted. We observed a linear correlation for the disap-pearance of the caged compounds, which suggested a first-order reaction,obtainedby linear least-squaresmethodology for a straight line. Photolysishalf-life values of caged compounds were calculated using eq 2
t1=2 ¼ 0:693
kpð2Þ
where kp is the first-order photolysis rate constant, obtained from the slopeof the linear plot of ln C versus irradiation time. Furthermore, the quantumyield for the photolysis of caged compounds was calculated using eq 3
ðΦpÞCG ¼ ðΦpÞact: �ðkpÞCGðkpÞact:
� ðFact:ÞðFCGÞ ð3Þ
where the subscripts “CG” and “act.” denote caged compound andactinometer, respectively. Potassium ferrioxalate was used as an acti-nometer (20). Φp is the photolysis quantum yield, kp is the photolysis rateconstant, and F is the fraction of light absorbed.
Preparative Photolysis. A solution of caged compound (2a-g)(0.05 mmol) in MeOH/HEPES (80:20) was irradiated using a 125 Wmedium-pressure Hg lamp filtered by suitable filters. The irradiation wasmonitored by TLC at regular intervals of time. After completion ofphotolysis, solvent MeOH/HEPES (80:20) was removed under vacuum,and the photoproducts (2,4-D and 4-(hydroxymethyl)-7-substituted
coumarin) were isolated by column chromatography using increasingpercentages of EtOAc in hexane as an eluant.
Laboratory Bioassay. Petri dishes of 9 cm diameter with aWhatmanno. 1 filter paper were used for bioassay experiments. Each Petri dish wasseparately moistened with 10 mL of the tested compound (9.05� 10-6 M).Control was similarly prepared, with the same amount of distilled water andfree 2,4-D. Ten seeds ofVigna radiata (MoongDal) were placed in each Petridish. Each treatment was replicated three times. After 10 days of incubation(all of the Petri dishes were exposed to daylight for 30 min each day), shootand root length were recorded. The data were analyzed by analysis ofvariance (ANOVA) followed by Duncan’s multiple-range test to delineatethe treatment means using SPSS computer software. The herbicidal activitywas assessed as the inhibition rate in comparison with the distilled water.
RESULTS AND DISCUSSION
Synthesis of Photoremovable Protecting Groups (1a-g). Wesynthesizedprotecting groups of (coumarin-4yl)methyl type havingdifferent substituents at the 7-position such as 4-bromomethyl-7-hydroxychromen-2-one (1a) (21), 4-bromomethyl-7-methoxychro-men-2-one (1b), acetic acid 4-bromomethyl-2-oxo-2H-chromen-7-yl ester (1c), (4-bromomethyl-2-oxo-2H-chromen-7-yl)carbamicacid ethyl ester (1d) (22), 7-amino-4-bromomethylchromen-2-one(1e), 4-bromomethyl-7-dimethylaminochromen-2-one (1f), and4-bromomethyl-7-diethylaminochromen-2-one (1g) (23) as shownin Scheme 1.
Synthesis of Caged Compounds of 2,4-D (2a-g). The caging of2,4-D using PRPG of coumarin derivatives was carried out bysimple esterification as outlined in Scheme 2. The freshly prepared4-(bromomethyl)-7-substituted coumarins (1a-g) were treatedwith2,4-D in the presence of K2CO3/KI in dry DMF for 3 h at 55 �C,resulting in the formation of corresponding caged compounds(2a-g) in excellent yield (90-95%) as summarized in Table 1.
All of the caged compounds were characterized by IR, 1H, 13CNMR, and mass spectral analysis. The IR spectra of cagedcompounds (2a-g) showed bands in the range of 1755-1770 cm-1 due to the stretching vibration of the newly formedester carbonyl group in addition to the carbonyl band ofcoumarinmoiety at around 1710 cm-1. 1HNMR spectra showedsignals corresponding to the ester methylene group (R-CH2) ataround δ 5.40 along with aromatic protons of 2,4-D at δ 6.65,7.04, and 7.17 ppm. In addition, we also observed characteristicsignals of the coumarin moiety at δ 6.10 (H-3, R,β-unsaturatedalkene) and 7.16-7.57 (aromatic protons).
The confirmation of the presence of the newly formed estergroup was further supported by the 13C NMR spectra signal ofthe ester carbonyl at δ 168 apart from the carbonyl signal of thecoumarin at δ 160.
Scheme 1. Synthesis of PRPG of Coumarin Derivatives (1a-g)a
Photophysical Properties of Caged Compounds of 2,4-D (2a-g).The photophysical properties of all the caged compounds wereinvestigated. The absorption and emission maxima, molar ab-sorptivities, Stokes’ shift, and fluorescence quantum yield of thecaged compounds (2a-g) are summarized in Table 1. TheUV-vis absorption spectra (Figure 1A) clearly show cagedcompounds (2a-g) have two strong intense absorption bandscentered in the region of λ = 321-381 nm and around λ =220 nm. The long wavelength absorption band corresponds tocoumarin chromophore (10) and the short wavelength is due to2,4-D (24). As anticipated, different substituents at the 7-positionon the coumarin moiety have great influence on the longerabsorption wavelength band of the caged compounds (Table 1).For example, caged compounds 2f and 2g with an electron-donating dialkylamino substituent at the 7-position of coumarin
showed a significant red shift of absorption maxima toward thevisible region combined with increasing extinction coefficient(Figure 1A).
Furthermore, to understand the fluorescence properties of thecaged compounds, the emission spectra (Figure 1B) were recordedby exciting the caged compounds (2a-g) at their correspondingabsorption maxima in MeOH/HEPES (80:20). The caged com-pounds exhibited strong fluorescence with maximum emissionwavelengths between 400 and 483 nm, and the magnitude of theStokes’ shift of all the caged compounds varies between 79 and169 nm. Furthermore, the caged compounds also showed mod-erate fluorescence quantum yield (0.59 < Φf < 0.03).
The photophysical studies revealed that caging of nonfluorescent2,4-D by PRPG of coumarin derivatives showed strong fluores-cence, large Stokes’ shift, and good fluorescence quantum yield.Hence, the above method could be useful to detect caged pesticideresidues at low level using a sensitive fluorescence technique.
Photolysis of Caged Compounds of 2,4-D (2a-g). Irradiation ofcaged compounds (2a-g) in MeOH/HEPES (80:20) buffer atdifferent wavelengths (310, 350, and 410 nm) resulted in con-trolled release of 2,4-D (Table 2). In each case, the photolysis wasstopped when conversion reached at least 95% (as indicated byHPLC). For all of the caged compounds mentioned in Table 2,the photolysis products (2,4-D and 4-(hydroxymethyl)-7-substi-tuted coumarins) were confirmed by isolating and matching their1H NMR spectra to those of authentic samples.
As a representative example we have shown in Figure 2A theHPLCof the photolysis of caged compound2a at regular intervalsof time.TheHPLCchart shows that as the irradiation time increaseswe can observe a gradual decrease of the peak at Rt=16.20 min,indicating the photocleavage of the caged compound 2a. On theother hand, we also note a gradual increase of two new peaks atRt = 3.14 and 2.42 min, corresponding to released 2,4-D and4-(hydroxymethyl)-7-hydroxycoumarin, respectively. Furthermore,wealsomonitored the course of photorelease of caged compound2gusing fluorescence (Figure 2B) and UV-vis (Supporting Informa-tion Figures S1 and S2) spectroscopy.
Similarly to previously discussed coumarinyl methyl cagedcompounds (10), the mechanism of the photocleavage of thecaged compounds of 2,4-D involves initial heterolysis of theC-Oester bond (photo-SN1) to produce an ion pair of coumarinylmethyl carbocation and a carboxylate anion of 2,4-D (Scheme 3).After ion pair separation in polar solvent, the methylenic carbo-cation is trapped by the solvent molecule to yield 4-(hydroxy-methyl)-7-substituted coumarins (3a-g). On the other hand, thecarboxylate anion abstracts a proton from the solvent to yield thecorresponding 2,4-D.
Scheme 2. Synthesis of Caged Compounds of 2,4-D (2a-g)
Table 1. Synthetic Yield, UV-Vis, and Fluorescence Data for the CagedCompounds (2a-g)
UV-vis fluorescence
caged
compd
synthetic
yielda (%)
λmaxb
(nm)
εc (104
M-1 cm-1)
λmaxd
(nm)
Stokes’
shifte (nm) Φff
2a 90 322 1.4 480 158 0.51
2b 93 321 1.4 400 79 0.13
2c 95 314 1.0 483 169 0.29
2d 94 327 2.1 414 87 0.41
2e 94 356 1.6 454 98 0.59
2f 91 370 2.0 476 106 0.18
2g 92 381 2.1 472 91 0.03
aBased on isolated yield. bMaximum absorption wavelength. cMolar absorptioncoefficient (M-1 cm-1) at the maximum absorption wavelength. dMaximumemission wavelength. eDifference between maximum emission wavelength andmaximum absorption wavelength. f Fluorescence quantum yield (error limit within(5%) was calculated using quinine sulfate as standard (Φf = 0.54 in 0.1 N H2SO4).
Figure 1. (A) UV-vis absorption spectra of the caged compounds (2a-g) in MeOH/HEPES (80:20) (3� 10-5 M). (B) Corrected emission spectra of thecaged compounds (2a-g) in MeOH/HEPES (80:20) (3� 10-5 M).
11848 J. Agric. Food Chem., Vol. 58, No. 22, 2010 Atta et al.
Linear regression analysis of the natural logarithm of theconcentration of caged compound (ln C) versus irradiation timeat different irradiation wavelengths is shown in Figure 3B. Thefirst-order photolysis rate constant (kp) values of caged com-pounds at different wavelengths (Table 2) indicates that substit-uents at the 7-position of the coumarinmoiety and the irradiationwavelength have pronounced effects on the release of 2,4-D.
Effect of Substituents at the 7-Position of the Coumarin Moiety
on the Release of 2,4-D.The results fromTable 2 clearly show thatdifferent substituents at the 7-position of the coumarinmoiety have
great influence on the ability of the PRPG to release 2,4-D. At310 nm, we note the efficiency of the caged compounds to release2,4-D increases as the electron-donating character of the substituentat the 7-position of the coumarin moiety increases (caged com-pound 2b with a stronger electron-donating -OMe group at the7-position showed 2 times higher value of photolysis rate constant(kp) compared to caged compound 2c with a moderate electron-donating -OCOCH3 substituent); this can be due to the stabiliza-tion of the intermediate coumarin-CH2
þ by an electron donorgroup (12). Caged compounds 2f and 2g, with strong electron-donating dialkyl amino substituents, showed 3-5 times lesser rateconstant (kp) values compared to 2b, because 2f and 2g have veryweak absorption at around 310 nm. The above substituent effectson the 2,4-D release can also be noted from the quantum yield (Φp)results.Figure 3A shows the percentage of release of pesticide 2,4-Dfrom different caged compounds (2a-g) after 20 h of irradiation at310 nm, and the release ranges from 37 to 96%.
Effect of Irradiation Wavelength on the Release of 2,4-D. Theinfluence of the irradiation wavelength on the extent of photo-cleavage of caged compounds (2a-g) in MeOH/HEPES buffer(80:20) is shown in Table 3. The time required for photocleavageof caged compounds increases as the irradiation wavelengthincreases. C-O bond dissociation energy is 86 kcal/mol, and aswe increase the wavelength, the excitation energy decreases from92 kcal/mol (310 nm) to 70 kcal/mol (410 nm). Hence, cagedcompound 2g has a relatively short half-life (t1/2=1325 min) at
Table 2. Photolytic Data of Caged Compounds (2a-g) at Different IrradiationWavelengths in MeOH/HEPES (80:20)
aMolar absorption coefficient (104 M-1 cm-1) at the irradiation wavelength.bRate constant (10-3 min-1) under photolytic conditions., cPhotochemical quan-tum yield (error limit within (5%).
Figure 2. (A)HPLCdata of photolysis of caged compound 2a at regular intervals of time: (i) 0 h; (ii) 2 h; (iii) 4 h; (iv) 6 h; (v) 4-(hydroxymethyl)-7-hydroxycoumarin;(vi) std 2,4-D. (B). Corrected emission spectra of the caged pesticide 2g at regular intervals of irradiation in MeOH/HEPES (80:20) (3� 10-5 M).
Scheme 3. Mechanism of the Photolysis of Caged Compounds (2a-g) To Release 2,4-D
310 nm compared to that at 410 nm (t1/2 = 9493 min) (Table 3).The incident photon flux (I0) at 310, 350, and 410 nm is 1.25 �1017, 9.66� 1016, and 4.12� 1016 photons s-1 cm-2, respectively.
Effect of Solvent on the Release of 2,4-D. To understand thesolvent effect on the rate of photorelease of 2,4-D, we carried outthe photolysis of caged compound 2g in solvents of differentpolarity and character such as aqueous mixtures of methanol,acetonitrile, and tetrahydrofuran using UV-vis light (310, 350,and 410 nm). The initial concentration of the caged compound 2gin each case was taken as 10-5 M, the release of pesticide 2,4-Dwas determined at regular intervals using HPLC, and the resultsare included in Table 4.
As anticipated, the character of the solvent has shown apronounced influence on the efficiency of photorelease of 2,4-D;the quantum yield for the photorelease of 2,4-D by 2g in methanoland acetonitrile was found to be higher compared to that in tetra-hydrofuran. This can be attributed due to (i) the formation of anionic intermediate (coumarin-CH2
þ) in Scheme 3 and (ii) thehydrogen bonding and polarity of the solvents (12). The above fact
was further confirmed by the increase in the quantum yield ofphotorelease of 2,4-D as the percentage of aqueous buffer inmethanol increases. We also carried out the photolysis of cagedcompound 2g in a water/ethanol (99:1) mixture and found muchhigher quantum yield for the photorelease of 2,4-D compared toother solvent systems used for the study. Figure 4A shows thepercentage of release of pesticide 2,4-D from 2g in different solventsafter 20 h of irradiation and the release ranges from 35 to 70%.
The stability of the caged compounds (2a-g) was evaluated bykeeping them in the dark in aqueous solvents for a period of30 days. We observed <10% decomposition of the caged com-pounds by HPLC.
Effect of pH on the Release of 2,4-D.To assess the role of pHonthe rate of release of 2,4-D, the solution containing cagedcompound (2g) at different initial pH values in the range of3.5-10.5 was irradiated using 310 nm. The photocleavage ki-netics at different pH values clearly indicates that the release of2,4-D is much slower at near-neutral pH in comparison to bothacidic and basic pH values (Figure 4B), because ester hydrolysis isshown to be accelerated by acid and base. Furthermore, therelease of 2,4-D on irradiation of 2g for 20 h varied from 37 to86% at different initial pH conditions (Figure 4B).
Herbicidal Activity. Preliminary results on the shoot and rootlength inhibition ofV. radiata by the caged compounds (2a-g) and4-(hydroxymethyl)-7-substituted coumarins (3a-g) obtained fromthe laboratory bioassay experiments are shown in Figure 5. Theresults indicate a 75-87% reduction in the shoot length of V.radiata with controlled release of 2,4-D using caged compounds(2a-g), whereas free 2,4-D showed a 90% reduction in shootlength. Among caged compounds, compounds 2a-c exhibitedbetter shoot length inhibition compared to compounds 2e-g,because caged compounds 2a-c showed efficient photorelease of2,4-D in comparison to compounds 2e-g (Table 2). We alsoobserved a similar trend in root length inhibition (78-90%) bythe caged compounds. Interestingly, we found that photoproducts4-(hydroxymethyl)-7-substituted coumarins (3a-g) also displayedinhibition of shoot length (36-62%) and root length (50-73%) ofV. radiata. Among the photoproducts, 4-(hydroxymethyl)-7-hy-droxycoumarin (3a) was found to have better inhibition activityagainst V. radiata.
The newly developed delivery device for the controlled releaseof 2,4-D based on PRPG of coumarin derivatives providesadvantages such as (i) greater control over the release by havinga suitable substituent at the 7-position of the coumarin moiety;(ii) hydrolytic stability to the pesticide; (iii) the fluorescence of thecaged pesticide mitigating low-level detection problems; and(iv) finally, the possibility of value-adding them to the formulationdue to their pesticidal activity. In the future we will report our
Figure 3. (A) Release of 2,4-D (%) versus irradiation time of the caged compounds (2a-g) in MeOH/HEPES (80:20) at 310 nm. (B) Plot of ln C versusirradiation time for the photolysis of 2g in MeOH/HEPES (80:20) at 310, 350, and 410 nm.
Table 3. Half-Life (t1/2) of Caged Compounds (2a-g) at Different IrradiationWavelengths in MeOH/HEPES (80:20)
t1/2 (min)
caged compd 310 nm 350 nm 410 nm
2a 155 800 -
2b 240 969 -
2c 503 1818 -
2d 319 1420 -
2e 703 1300 -
2f 859 3380 11588
2g 1325 3284 9493
Table 4. Photolytic Data of Compound 2g at Different IrradiationWavelengthsin Different Solvent Systems
aMolar absorption coefficient (104 M-1 cm-1) at the irradiation wavelength.bRate constant (10-3 min-1) under photolytic conditions. cPhotochemical quantumyield (error limit within (5%).
11850 J. Agric. Food Chem., Vol. 58, No. 22, 2010 Atta et al.
results on controlled release of 2,4-D in a soil medium using PRPGunder sunlight.
ACKNOWLEDGMENT
We thank DST FIST for 400 MHz NMR and the IndianAssociation for the Cultivation of Science, Kolkata, for HRMSanalysis.We thankProf. S.Dasgupta andProf. S.Nanda for theirhelp in carrying out laboratory bioassay.
Supporting Information Available: 1H NMR and 13C NMR
spectra of compounds 2a-2g and absorption spectrum of com-
pound 2g in MeOH/HEPES (60:40) solvent system under different
irradiation times (Figure S1); absorption spectrum of compound 2g
tion times (Figure S2); plot of ln C versus irradiation time of caged
compound 2g at 310 nm in different solvent systems and in different
pH conditions (Figure S3); effect of tested compounds on shoot and
root length growth on Vigna radiata (Table S1). This material is
available free of charge via the Internet at http://pubs.acs.org.
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Figure 4. (A)Release of 2,4-D (%) on irradiation of 2g at 310 nm in different solvent systems: (S1)methanol; (S2)methanol/HEPES (80:20); (S3)methanol/HEPES (60:40); (S4) methanol/HEPES (50:50); (S5) methanol/HEPES (40:60); (S6) acetonitrile/HEPES (80:20); (S7) THF/water (50:50); (S8) ethanol/water (1:99). (B) Release of 2,4-D (%) on irradiation of 2g at 310 nm in different pH conditions.
Figure 5. Effect of tested compounds on shoot and root length growth on Vigna radiata. Vertical bars show standard errors. Bars with different letters showsignificant difference (Pe 0.05) as determined byDuncan’smultiple-range test. C1 andC2 indicate controlled experiments using distilled water and free 2,4-D,respectively.
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Received for review July 19, 2010. Revised manuscript receivedOctober
